Note that I at least will be marking them using the "Japanese standards", meaning that a mediocre story will probably still be better than most stories in western games whereas superior gameplay may still me inferior to the average western game.

So here's a few VNs to start off:

Fate Stay Night - Hentai

Art - Decent, nothing spectacular here, but nothing bad either

Music - Nothing stands out, which is a bad thing in my book

Story - Had a few changes of heart on this matter. Final verdict: Kinda weak.

Other notes - My first VN. L-am citit dupa ce am vazut animeul and even then the first route left me rather disappointed. After seeing the true potential of the Fate universe realised in the Fate/Zero series my opinion of it plummeted even more. My advice don't bother with it. Go watch Fate/Zero since it's a true masterpiece and read the Type Moon wiki for some more info.

Sengoku Rance - Hentai

Art - Good

Music - Awesome, some of the best battle and boss themes ever

Gameplay - Very good! I mean it. Really good Turn Based Strategy that's not overly complicated and inaccessible to newcomers to the genre.

Story - You're gonna love the main char even if you hate him. The story is as simple as it can be, but Rance can making even the most stupid things enjoyable.

Other notes - If you've played any other Rance game then you know what I'm talking about. The game would be worth playing even just for the main character. Add some great music and gameplay to that and you get one of the best VNs ever.

Rewrite - NOT Hentai

Art - Excellent

Music - Amazing

Gameplay - Consists in finding some hidden bubbles on a map which I found rather annoying.

Story - Masterpiece. Go read it. There no hentai, almost no gameplay, and limited romantic development so most of the effort went into making a great story. And it succeeded.

Other notes - This is one of the absolutely must read VNs. Alas, there is one misfortune that plagues it and that is inconsistency. The routes are written by several authors so there's an inconsistency in quality and story between routes belonging to different authors.

Yosuga no Sora - Hentai

Art - Good enough

Music - Nothing stands out

Story - Mediocre

Other notes - Only went there for the twincest, so I only read Sora's route because of the twincest. It also has twincest...................twincest

Katawa Shoujo - Hentai

Art - Acceptable

Music - Very Good. Might not seem impressive at first but once you get into the story you'll come to appreciate it. Also hearing the title music after finishing a route is known to give the sensation of having your chest crushed.

Story - A masterpiece among masterpieces. Stories with no fantasy, action, competitions or anything of the sort start off with a really big minus for me, yet this VN managed to make an impression on me unlike anything else.

Other notes - This VN is as good as they say and should be read by anyone in my opinion. It is also (unfortunately) quite short so you can safely push it up if it's somewhere far on the to-read list.

Clannad - NOT Hentai

Art - Good

Music - Decent

Story - Very Good

Other notes - I'd say the anime did the VN justice. If you liked it go ahead and play the game as well.

Monster Girl Quest - Hentai, lots of hentai

Art - Very good

Music - Very good

Gameplay - Reasonably good. While many parts are scripted and battles are short they do offer a small amount of enjoyment, especially that surrender button.

Story - Surprisingly good. Many come for the hentai and stay for the story... and the hentai.